The concrete pumps commonly used in modern building construction are of the ram piston type, an example of which is shown in U.S. Pat. No. 3,327,641. Such pumps utilize rams whose ram cylinders are filled with concrete from a hopper during the back stroke, and during the forward stroke, push the concrete into a pipeline which leads to the form. Typically, a pair of rams is utilized, one moving rearward and taking in concrete, while the other moves forward to pump the concrete into the pipeline. Each ram requires a pair of valves for its operation. One is the hopper valve which leads to the hopper. It is open when concrete is flowing from the hopper to the ram cylinder associated with the particular hopper valve. It closes during the forward stroke of the ram piston to prevent concrete from being pumped back into the hopper. The other valve associated with the one ram is the pipeline valve which is positioned in front of the ram. The pipeline valve is open during the forward stroke of the ram piston to allow the concrete to enter the discharge conduit and is closed during the back stroke to prevent concrete from returning through the pipeline to the ram. Where concrete is pumped to high levels, large pressures are encountered. These high pressures plus the highly abrasive nature of the concrete results in large wear on the valve. The wear is extremely hard on the pipeline valve since it is subjected to large pressures during both its opened and closed positions.
A typical prior art valve such as that shown in U.S. Pat. No. 3,327,641, utilizes the general form of a T-pipe section in series with the pipeline or hopper. The elongated leg of the T served as the valve cylinder and a valve piston moved through this cylinder to open and close the valve. The piston comprised a front bullet-shaped member of rubber, followed by a thick steel rod. The steel rod had a pair of grooves for holding packing glands. In the case of the pipeline valves, the piston was designed so that during the open valve position, when the bullet was within the cylinder, the pumping pressures would expand the rubber. This would prevent the liquid portion of the concrete, generally referred to as sap, from passing by the piston. The packing glands served as a second barrier to sap leakage during the open valve position, and as the primary barrier when the valve was closed.
The aforedescribed valves often failed to work properly after a brief period of difficult pumping conditions. Typically, after a few hundred cubic yards of concrete had been pumped, the piston would "hang" on the cylinder walls, that is, the piston would not move. The pump would have to be stopped and the piston freed. Hanging and erratic performance would continue for the remaining life of the packing, which was only about 1,000 cubic yards of pumped concrete. The valve assembly would then have to be disassembled and new packing installed to prevent excessive sap leakage. After several thousand cubic yards, the rubber bullet was so worn that the entire piston had to be replaced.
The erratic performance and limited life of the packing glands is largely due to their exposure to the concrete sap, which quickly hardens. Many valve pistons have incorporated grease fittings for continually supplying grease to the packing glands. However, the grease fittings do not substantially extend the lifetime or eliminate erratic performance after several hundred cubic yards have been pumped. They have little effect on the wear of the rubber bullet. The erratic performance and need for repeated changes of the packing glands is costly in that it limits production. In addition, the pistons are costly, and their limited life of only several thousand cubic yards of pumped concrete results in high maintenance costs.
One attempt to provide a more trouble-free valve for a concrete pump is found in U.S. Pat. No. 3,529,804. The valve described therein utilizes the standard T-pipe section communicating with a cylinder in conjunction with a front valve closing portion and provides a tapered expander member behind the valve closing portion with a thick cup shaped seal having a large downwardly extending lip portion placed thereover. A pressure plate is placed behind the seal and when wear was found on the periphery of the seal, the pressure plate could be tightened against the seal, expanding the seal and providing a new peripheral sealing area. However, this valve also proved unsatisfactory, as the implementation of the expandable seal with its extending lip portion, designed to distribute the expansion forces to a large area of the seal, has resulted in an extensive dragging of the seal and particularly the large lip portion during the rearward stroke of the valve piston. In addition, due to the positioning of the valves on the concrete pump, the access to the tightening means for expanding the seal on such valves is somewhat awkward, which also makes difficult the task of disassembly for replacement of worn parts, such as the piston cylinder which is very susceptible to wear and must often be replaced.